Rocket designers select a type, density, and shape propellant grain to achieve the needed thrust, duration, and burn characteristics desired for a given performance profile. Often, the best grain selection can yield a rocket motor that has natural ignition characteristics different from those the designer would prefer, considering other requirements such as the needs of the launcher system. Thus, it is desirable to be able to tailor the ignition of a propellant grain so as to produce modified start up characteristics.
For example, in a vertical launch system on board ships, several rockets may share an exhaust plenum and diverter that could be damaged by the sudden blast from a quick ignition of a rocket. In this case, the propellant grain sometimes is coated with an inhibitor to slow down propagation of the flame spread and avoid sudden pressure rises. On the other hand, a design goal that emphasizes fast response may require a coating on the propellant that enhances and accelerates ignition.
Other ignition characterizing systems usually paint some material on the surface of the propellant grain to inhibit ignition. But these systems have numerous difficulties. Painting is labor intensive and quality control is hard to maintain. Also, the chemical composition of the grain may be affected, especially during long term storage, as various constituents of the propellant react with or migrate into the inhibitor. Conversely, the coating may introduce contaminants that migrate into the grain and alter its behavior. Since grains are typically hollow, having a small diameter burning surface inside, they are hard to inspect for coating quality or for long term degradation. Similar problems are encountered with ignition enhancing coatings.